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Thursday, August 12, 2021

SPC Aug 12, 2021 1630 UTC Day 1 Convective Outlook

SPC 1630Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1128 AM CDT Thu Aug 12 2021 Valid 121630Z - 131200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON/EVENING FROM EAST CENTRAL OH INTO WEST CENTRAL AND NORTH CENTRAL PA... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THROUGH TONIGHT FROM CENTRAL IL TO NORTHEAST KS... ...SUMMARY... Damaging winds will be the main threat with clusters of storms this afternoon/evening from eastern Ohio into Pennsylvania. More isolated severe storms with damaging winds will also be possible into New York and Vermont, and from northeast Kansas to central Illinois. ...OH/PA/NY/VT this afternoon/evening... Regional 12z soundings and VWP observations show a belt of 35-50 kt flow near 700 mb/3 km AGL over the lower Great Lakes, likely related to remnant MCVs from prior convection. These embedded perturbations will move east-northeastward around the far southeastern periphery of the deep midlevel low over Hudson Bay. The development of scattered cells/clusters is expected by early afternoon from central NY toward VT in conjunction with weak ascent and differential heating on the east edge of the thicker clouds over western NY, with an attendant threat for occasional wind damage through the afternoon. Other storms are expected to form this afternoon from northwestern PA into northeastern OH, in association with remnant MCVs over IN and Lake Erie, and along a zone of remnant outflow and differential heating. Clusters evolving into line segments will be capable of producing damaging outflow gusts of 55-65 mph in an environment with MLCAPE of 2500-3500 J/kg, DCAPE in excess of 1000 J/kg, and 30+ kt westerly flow in the 700-500 mb layer. ...Northern/central IL to northeastern KS through tonight... A regime of cyclonic flow aloft will persist from the northern Plains to the Great Lakes, as subtle/embedded speed maxima rotate eastward through the base of the trough. An associated reinforcing frontal surge is moving southeastward across NE/IA/WI, while a diffuse pre-frontal outflow boundary extends eastward from southern IA to northern IL/IN. Despite weakening low-level warm advection, convection has expanded in coverage along this remnant boundary from southeast IA into east central IL. Isolated downbursts and/or marginally severe hail could accompany these storms, especially if they persist into the afternoon as the low levels warm/moisten to the south. North of the ongoing storms, there is growing concern that clouds and/or rain-cooled air may be sufficient to interfere with later storm development along the reinforcing front from southeast IA into northwest IL. MLCAPE of 3500-5000 J/kg is expected along and south of the boundaries based on mid 70s boundary-layer dewpoints beneath 8 C/km midlevel lapse rates. The thermodynamic environment will favor intense updrafts and precipitation-loaded downdrafts capable of producing wind damage with outflow winds of 60-70 mph, as well as isolated hail near 1 inch diameter. This corridor will be along the southern fringe of the stronger flow aloft and transient supercell structures will be possible, but the primary convective mode should favor clusters. The evening storms will spread southeastward with the front and storm-scale outflow, and additional development is expected overnight as warm advection increases from northeast KS into northern MO on the nose of a modest southwesterly low-level jet. ..Thompson/Bentley.. 08/12/2021 Read more LIVE: spc.noaa.gov
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